P.CHAITHANYAKUMAR, T.VARAPRASAD/ International Journal of Engineering Research and
Applications (IJERA)
ISSN: 2248-9622
www.ijera.com
Vol. 2, Issue 3, May-Jun 2012, pp.1088-1094
Design of Unified Power Quality Conditioner (UPQC) to Improve
the Power Quality Problems by Using P-Q Theory
P.CHAITHANYAKUMAR * T.VARAPRASAD**
*PG Student
Department Of Electrical & Electronics Engineering
Sri Venkatesa Perumal College of Engineering & Technology
**Assistant Professor
Department Of Electrical & Electronics Engineering
Sri Venkatesa Perumal College of Engineering & Technology
AbstractThis paper presents a Design of a Unified Power
Quality conditioner (UPQC) to improve the power
quality problems by using P-Q theory. The design of
UPQC connected to three phase four wire system
(3P4W). The neutral of series transformer used in the
fourth wire for the 3P4W system. The neutral
current that may flow toward transformer neutral
point is compensated by using a four-leg voltage
source inverter topology for shunt part. The series
transformer neutral will be at virtual zero potential
during all operating conditions.
In this simulation we observe the power quality
problems such as unbalanced voltage and current,
harmonics by connecting non linear load to 3P4W
system with Unified Power Quality conditioner. And
also calculating the THD (1.46%) and active power
and reactive power. A new control strategy is
proposed to the control algorithm for series APF is
based on unit vector template generation to
compensate the current unbalance present in the load
currents by expanding the concept of single phase PQ theory. The P-Q theory applied for balanced three
phase system. And also be used for each phase of
unbalanced system independently.
sophisticated equipment/loads at transmission and
distribution level has increased considerably in recent
years due to the development in the semiconductor
device technology. The equipment needs clean power in
order to function properly. At the same time, the
switching operation of these devices generates current
harmonics resulting in a polluted distribution system.
The power-electronics-based devices have been used to
overcome the major power quality problems [1], [2].
A 3P4W distribution system can be realized by
providing the neutral conductor along with the 3 power
lines from generation station. The unbalanced load
currents are very common and an important Problem in
3P4W distribution system. To improve the power quality
by connecting the series active power filter (APF) and
shunt active power filter (APF).The unbalanced load
currents are very common and yet an important problem
in 3P4W distribution system. This paper deals with the
unbalanced load current problem with a new control
approach, in which the fundamental active powers
demanded by each phase are computed first, and these
active powers are then redistributed equally on each of
the phases. UPQC is a multifunction power conditioner
that can be used to compensate various voltage
Index Terms- Active Power Filter(APF), four-leg disturbances of the power supply, to correct voltage
voltage –source inverter(VSI) structure, three-phase fluctuation, and to prevent the harmonic load current
four-wire (3P4W)system, unified power quality from entering the power system.
conditioner(UPQC).
II. PROPOSED 3P4W DISTRIBUTION
SYSTEM UTILIZING UPQC
I.INTRODUCTION
The power electronic devices due to their inherent
non-linearity draw harmonic and reactive power from
the supply. In three phase systems, they could also cause
unbalance and draw excessive neutral currents. The
injected harmonics, reactive power burden, unbalance,
and excessive neutral currents cause low system
efficiency and poor power factor. The design of shunt
active filter is described in [1].The use of the
Generally, a 3P4W distribution system is realized by
providing a neutral conductor along with three power
conductors from generation station or by utilizing a
three-phase Δ–Y transformer at distribution level. Fig.
2.1 shows a 3P4W network in which the neutral
conductor is provided from the generating station itself,
whereas Fig.2.2 shows a 3P4W distribution network
considering a Δ–Y transformer.
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P.CHAITHANYAKUMAR, T.VARAPRASAD/ International Journal of Engineering Research and
Applications (IJERA)
ISSN: 2248-9622
www.ijera.com
Vol. 2, Issue 3, May-Jun 2012, pp.1088-1094
Assume a plant site where three-phase three-wire
UPQC is already installed to protect a sensitive load and
to restrict any entry of distortion from load side toward
utility, as shown in Fig.2.3. If we want to upgrade the
system now from 3P3W to 3P4W due to installation of
some single-phase loads and if the distribution
transformer is close to the plant under consideration,
utility would provide the neutral conductor from this
transformer without major cost involvement.
Fig 2. 1: 3P4W distribution system: neutral provided from generation station.
Fig 2.2 : 3P4W distribution system: neutral provide from
transformer.
Fig 2.3: 3P3W UPQC structure.
Recently, the utility service providers are putting function as a controlled voltage source. If we could use
more and more restrictions on current total harmonic the neutral of three-phase series transformer to connect a
distortion (THD) limits, drawn by nonlinear loads, to neutral wire to realize the 3P4W system, then 3P4W
control the power distribution system harmonic system can easily be achieved from a 3P3W system (Fig.
pollution. At the same time, the use of sophisticated 2.4). The neutral current, present if any, would flow
equipment/load has increased significantly, and it needs through this fourth wire toward transformer neutral
clean power for its proper operation. Therefore, in future point. This neutral current can be compensated by using
distribution systems and the plant/load centers, a split capacitor topology [2], [9], [10] or a four-leg
application of UPQC would be common. Fig.2. 4 shows voltage-source inverter (VSI) topology for a shunt
the proposed novel 3P4W topology that can be realized inverter [2], [11].
from a 3P3W system. This proposed system has all the
advantages of general UPQC, in addition to easy
In this paper, the four-leg VSI topology is
expansion of 3P3W system to 3P4W system.
considered to compensate the neutral current flowing
toward the transformer neutral point. A fourth leg is
Thus, the proposed topology may play an important added on the existing 3P3W UPQC, such that the
role in the future 3P4W distribution system for more transformer neutral point will be at virtual zero potential.
advanced UPQC based plant/load center installation, Thus, the proposed structure would help to realize a
where utilities would be having an additional option to 3P4W system from a 3P3W system at distribution load
realize a 3P4W system just by providing a 3P3W supply. end. This would eventually result in easy expansion from
As shown in Fig.2.3, the UPQC should necessarily 3P3W to 3P4W systems.
consist of three-phase series transformer in order to
connect one of the inverters in the series with the line to
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P.CHAITHANYAKUMAR, T.VARAPRASAD/ International Journal of Engineering Research and
Applications (IJERA)
ISSN: 2248-9622
www.ijera.com
Vol. 2, Issue 3, May-Jun 2012, pp.1088-1094
Fig 2.4: Proposed 3P4W system realized from a 3P3W system utilizing UPQC.
III.DESIGN OF UPQC CONTROLLER
=
(3.4)
The control algorithm for series APF is based on unit
vector template generation scheme [8].Where as the
control strategy for shunt APF is discussed in this In addition, for phase c, the load voltage and current in
section. Based on the load on the 3P4W system, the α–β coordinates can be represented by π/2 lead as
current drawn from the utility can be unbalanced. In this
paper, the concept of single phase P-Q theory [9], [10].
=
=
According to this theory, a single phase system can be
defined as a pseudo two-phase system by giving π/2 lead (3.5)
or π /2 lag that is each phase voltage and current of the
original three phase systems.
=
(3.6)
These resultant two phase systems can be represented
in α-β coordinates, and thus P-Q theory applied for
balanced three phase system [11] can also be used for
each phase of unbalanced system independently. The
actual load voltages and load currents are considered as
α-axis quantities, whereas the π/2 lead load or π/2 lag
voltages and π/2 lead or π/2 lag load currents are
considered as β-axis quantities. In this paper, π/2 lead is
considered to achieve a two-phase system for each
phase. The major disadvantage of p–q theory is that it
gives poor results under distorted and/or unbalanced
input/utility voltages [4], [5]. In order to eliminate these
limitations, the reference load voltage signals extracted
for series APF are used instead of actual load voltage.
For phase a, the load voltage and current in α–β
coordinates can be represented by π/2 lead as
=
=
=
(3.1)
By using the definition of three-phase p–q theory for
balanced three-phase system [3], the instantaneous
power components can be represented as
Instantaneous active power
=
(3.7)
Instantaneous reactive power
=
(3.8)
Considering phase a, the phase-a instantaneous load
active and instantaneous load reactive powers can be
represented by
=
Where
(3.9)
=
+
(3.10)
=
+
(3.11)
(3.2)
In (3.10) and (3.11),
and
represent the dc
components that are responsible for fundamental load
Where
represents the reference load voltage and active and reactive powers, whereas
and
represent
represents the desired load voltage magnitude.
the ac components that are responsible for harmonic
powers. The phase-a fundamental instantaneous load
Similarly, for phase b, the load voltage and current in active and reactive power components can be extracted
from
and
, respectively, by using a low pass
α–β coordinates can be represented by π/2 lead as
filter.
=
=
Therefore, the instantaneous fundamental load active
and reactive power for phase a is given by
(3.3)
=
(3.12)
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P.CHAITHANYAKUMAR, T.VARAPRASAD/ International Journal of Engineering Research and
Applications (IJERA)
ISSN: 2248-9622
www.ijera.com
Vol. 2, Issue 3, May-Jun 2012, pp.1088-1094
since the utility should not supply load reactive power
demand. In the above matrix, the α- axis reference
Similarly, the fundamental instantaneous load active and compensating current represents the instantaneous
fundamental source current, since α-axis quantities
reactive powers for phase b is given by
belong to the original system under consideration and the
=
(3.14)
β-axis reference compensating current represents the
=
(3.15)
current that is at π/2 lead with respect to the original
system.
Therefore,
Instantaneous fundamental load active and reactive
=
(3.21)
power for phase c
=
(3.16)
Similarly, the reference source current for phases b
and c can be estimated as
=
(3.17)
=
(3.22)
=
(3.13)
Since the load current drawn by each phase may be
different due to different loads that may be present inside
plant, therefore, the instantaneous fundamental load
active power and reactive power demand for each phase
may not be the same. In order to make this load
unbalanced power demand, seen from the utility side, as
a perfectly balanced fundamental three-phase active
power the unbalanced load power should be properly
redistributed between utility, UPQC, and load, such that
the total load seen by the utility would be linear and
balanced load. The unbalanced or balanced reactive
power demanded by the load should be handled by a
shunt APF. The aforementioned task can be achieved by
summing instantaneous fundamental load active power
demands of all the three phases and redistributing it
again on each utility phase, i.e., from (3.12), (3.14), and
(3.16),
=
+
+
(3.18)
=
(3.19)
Equation (3.19) gives the redistributed per-phase
fundamental active power demand that each phase of
utility should supply in order to achieve perfectly
balanced source currents. From (3.19), it is evident that
under all the conditions, the total fundamental power
drawn from the utility but with perfectly balanced way
even though the load currents are unbalanced. Thus, the
reference compensating currents representing a perfectly
balanced three-phase system can be extracted by taking
the inverse of (3.9)
=
(3.23)
The reference neutral current signal can be extracted
by simply adding all the sensed load currents, without
actual neutral current sensing, as
=
(3.24)
=
(3.25)
The proposed balanced per-phase fundamental active
power estimation, dc-link voltage control loop based on
PI regulator, the reference source current generation as
given by (3.21)–(3.23),and the reference neutral current
generation are shown in Fig 4.3 to Fig 4.7, respectively.
IV. SIMULATION RESULTS
The simulation results for the proposed 3P4W system
realized from a 3P3W system utilizing UPQC are shown
in below figures 4.2 to 4.10.Utility voltage are assumed
to be distorted with voltage THD of 14.03 % is shown in
Figure.4.7.The distorted voltage profile is shown in
figure.4.2(A). The resulting load current profile shown in
figure.4.4 (B) has THD of 12.10% in Figure.6.10.
The UPQC should maintain the voltage at load bus at
a desired value and free from distortion. The plant load
is assumed to be the combination of a balanced threephase diode bridge rectifier followed by an R-L load,
which acts as a harmonic generating load, and three
different single-phase loads on each phase, with different
=
(3.20) load active and reactive power demands. The shunt APF
is turned on first at time t=0.1sec, such that it maintains
the dc-link voltage at a set reference value, here
In (3.20),
is the precise amount of per-phase V=220V.At time t=0.2sec (is shown in figure.4.).
active power that should be taken from the source in
The series active power filter injects the required
order to maintain the dc-link voltage at a constant level compensating voltages through series transformer,
and to overcome the losses associated with UPQC. The making the load voltage free from distortion (THD =
oscillating instantaneous active power should be 1.46%) and at a desired level as shown in figure.4.2 (B)
exchanged between the load and shunt APF. The in load voltage. The series active power filter injected
reactive power term ( ) in (3.20) is considered as zero, voltage profile is shown in figure.4.2(C).
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P.CHAITHANYAKUMAR, T.VARAPRASAD/ International Journal of Engineering Research and
Applications (IJERA)
ISSN: 2248-9622
www.ijera.com
Vol. 2, Issue 3, May-Jun 2012, pp.1088-1094
The compensated source currents shown in figure.4.4
(A) are perfectly balanced with the THD of 2.26% is
shown in figure 4.9 .The compensating current injected
through the fourth leg of the shunt APF is shown in
figure.4.4(C).The load neutral current profile is shown in
figure.4.5 and figure.4.6.In figure.4.3, the shunt APF
effectively compensates the current flowing toward the
transformer neutral point. Thus, the series transformer
neutral point is maintained at virtual zero potential is
shown in figure 4.3(B).
Figure.4.1 Simulation Block Diagram of 3P4W system realized from a 3P3W system utilizing UPQC.
Figure.4.2: (A) Utility Voltage (Vs_abc), (B) Load
Voltage (Vl_abc) and (C) Injected Voltage (Vinj_abc).
Figure.4.4 (A) Source Current (Is_abc), (B) Load Current
(Il_abc) and (C) Shunt Compensating Current (Ish_abc).
Figure.4.5: Shunt neutral compensating current (i Sh_n).
Figure.4.3 :( A) Dc-link voltage (vdc), and (B) neutral
current flowing towards series transformer (i Sr_n).
Figure.4.6: Current flowing through load neutral wire
(iL_n).
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P.CHAITHANYAKUMAR, T.VARAPRASAD/ International Journal of Engineering Research and
Applications (IJERA)
ISSN: 2248-9622
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Vol. 2, Issue 3, May-Jun 2012, pp.1088-1094
Figure.4.8: Load Voltage THD=1.46%.
Figure.4.7: Distorted Voltage THD = 14.03%.
Figure.4.10: Load Current THD = 12.10%.
Figure.4.9: Source Current THD = 2.26%.
V. CONCLUSION
The design of a unified power quality conditioner
(UPQC) connected to 3P4W distribution system has
been presented in this project. Where upqc is installed to
compensate the different power quality problems, which
may play an important role in future upqc based
distribution system. The simulation results shows that
the distorted and unbalanced load currents seen from the
utility side act as perfectly balanced source currents and
are free from distortion. Here we can absorb the power
quality problems like voltage and current unbalanced
and reduced the total harmonic distortion (THD) of
3P4W system utilizing 3P3W system to connect the
UPQC. The neutral current that may flow toward the
transformer neutral point is effectively compensated
such that the transformer neutral point is always at
virtual zero potential.
Proposed model for the UPQC is to compensate
input voltage harmonics and current harmonics caused
by non-linear load. The work can be extended to
compensate the supply voltage and load current
imperfections such as sags, swells, interruptions, voltage
imbalance, flicker, and current unbalance. Proposed
UPQC can be implemented using Multi converterunified power quality conditioning system (MC-UPQC).
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